I wrote a solution to this weeks ago that re-directs a sub-domain to the mobile version of Oxwall, but if you change the link for the /mobile-version website to whatever you need the sub-domain to go to, this should work:
EWT: How to create a sub-domain redirect on Oxwall
Make sure to turn into the correct sub-domain url, and to turn to the right page the sub-domain re-directs to. If you're using a host with a cpanel, go to sub-domains and enter "m" and create, it will make a folder called "m" in your public_html directory and you can add the code in the link provided to your Oxwall .htaccess or to a new .htaccess in "m."
~Jake
www.ewtnet.us